In the manufacture of ceramic sheathed-element glow plugs from ceramic composite materials, amorphous SiOC ceramics (precursor ceramics) are obtained by the pyrolysis of elementorganic precursors. Advantages of this precursor-thermolysis method in comparison with conventional manufacturing methods for ceramics (e.g., sintering) are the considerably lower process temperatures and the easy workability and malleability of the elementorganic precursors such as polysiloxane resins for example.
To manufacture stable ceramic bodies from organic silicon polymers, temperatures of approximately 800° C. already suffice, while sintering powder for the most part only yields mechanically stable bodies at sintering temperatures above 1200° C. Thus the manufacture of ceramic solids from filled organic silicon polymers requires considerably lower temperatures than the sintering of ceramic powders. Such a method is known from published European patent document EP 0 412 428, for example. In this instance, a metallic filler, which reacts with the decomposition products resulting in the pyrolysis of the polymer compounds, is admixed to the starting polymers. Pyrolysis takes place at a temperature in the range of 600 to 1800° C. and frequently in an inert-gas atmosphere. Filler components used, among others, are, e.g., chromium, molybdenum, silicon and intermetallic compounds of representatives of the fourth to sixth subgroup of the periodic system with boron, silicon or aluminum. These fillers are required since otherwise shrinkage cracks and an excessive number of pores are formed during pyrolysis. With the aid of these fillers it is possible to set in a precise manner certain properties such as, e.g., the coefficient of thermal expansion, thermal conductivity or the specific electrical resistance of the composite.
In the manufacture of a ceramic composite material from a precursor ceramic, in which, for example, a polysiloxane, i.e., a polymer on the basis of Si, C, O and H, is used, it is therefore possible by selecting the appropriate fillers to precisely tailor the electrical or physical property profile of the ceramic composite material resulting from the pyrolysis to the respective requirement profile, e.g., of a ceramic sheathed-element glow plug. In particular, it is possible in this manner to set the electrical conductivity from very conductive to insulating.
The mechanical properties of precursor ceramics (e.g., bending strength<350 MPa, fracture toughness Klc<MPa√{square root over (m)}), however, can be improved only to a limited extent by fillers that have a low aspect ratio, i.e., a low aspect ratio between the length and the diameter of <5. Fillers having a high aspect ratio of >10, such as fibers for example, offer considerably better results.
Due to gaseous decomposition products formed during pyrolysis, the precursor ceramic is porous. The size of the pores lies in the range of approximately 300 to approximately 800 nm. The bonding of whiskers, typically having a diameter of approx. 1 μm and lengths of approx. 100 μm, and fibers to the precursor ceramic is deteriorated by these pores since the pores reduce the effective whisker-fiber surface adhering in the precursor ceramic. The reinforcing effect of whiskers and fibers in components made of precursor ceramic is thereby reduced. The use of carbon nanotubes/fibers for reinforcing precursor ceramics offers advantages due to the considerably smaller dimensions at the same aspect ratio. Typical dimensions of carbon nanotubes are diameters of approximately 20 to approximately 120 nm and a length of approximately 0.5 to approximately 200 μm. In addition, carbon nanotubes are very insensitive to mechanical stresses in manufacturing (e.g., mixing, kneading, grinding, sifting) the composite made of the elementorganic precursor, fillers and the carbon nanotubes, since in a breakup of the carbon nanotubes, e.g., a bisection, the aspect ratio is still large enough.
Due to their outstanding properties, carbon nanotubes as fiber reinforcement in composite materials are of great interest and are already used in plastic composites and sinter ceramics.
Published International patent document WO 01/92381, for example, describes a method for forming a composite material from embedded nanofibers in a polymer matrix. The method comprises the introduction of nanofibers into a plastic matrix with the formation of agglomerates and the uniform distribution of the nanofibers by exposing the agglomerates to hydrodynamic stresses. Likewise disclosed is a nanofiber-reinforced polymer composite system, which has a plurality of nanofibers embedded in polymer matrices. A method for manufacturing fibers reinforced by nanotubes comprises the admixture of a nanofiber into a polymer and inducing an orientation of the nanofiber, which allows the latter to be used to improve the mechanical, thermal and electrical properties.
Published International patent document WO 02/18296 discloses a ceramic matrix-nano-composite material having improved mechanical properties. This is made up of a filler made of carbon nanotubes and a ceramic matrix which is fabricated from a nano-crystalline ceramic oxide. By sintering the article formed from this, it is possible to obtain ceramic materials of improved fracture toughness.
The properties of carbon nanotubes of the type MWNT (multiwall carbon nanotubes) are as follows:
Thermal conductivity: >2000 W/mK
Tensile strength: >10 GPa
Young's modulus: up to 1200 GPa
Electric conductivity: Semiconductor or metallic
Aspect ratio: 100-1000
The number of manufacturers of carbon nanotubes is steadily increasing. Commercial production at over 100 t/year has been achieved in the meantime, which results in markedly falling prices for such carbon nanotubes.
An objective of the present invention is to provide a method to improve the properties of composite materials made from precursor ceramics.